It is known that the rotors of these machines often support considerable axial thrusts due to differences in pressure which arise on either side of the wheels, to variations in quantity of movement of fluids conveyed and, in some cases, to the weight or a fraction of the weight of the rotor itself.
Because of this, pumps usually contain devices enabling compensation of the axial thrust exerted by the fluid on the rotor.
These devices are likely to use all the surfaces on the front face (to the side of the vanes) and inversely on the rear face or back of the centrifuge wheels as surfaces on which fluid is to circulate to compensate the axial impingement of forces exerted on the rotating parts. As is known, the back of the last centrifuge wheel, for this reason known as the “balancing piston” is used more particularly for achieving this balancing.
FIGS. 1 and 2 illustrate such a known thrust balancing device arranged between a wheel 11 and a stator 12.
The turbopump of FIG. 1 essentially comprises a rotating assembly placed around a central shaft 26, of reduced length, and comprising a single impeller 11 of a single-stage centrifuging pump mounted on the shaft 26, in the median part of the latter, as well as two turbine wheels 22, mounted on the shaft 26 in the rear part of the latter.
The turbine wheels 22, mounted overhanging the rear of the shaft 26, drive the latter in rotation under the action of a hot gas flow applied to the periphery of the turbine wheels 22, from a gas inlet core 24.
FIG. 1 illustrates for the single-stage pump an impeller 11 of open type, with vanes 6 receiving the working fluid via a suction channel 2 and repelling the pressurised working fluid via a pressure passage 3.
The working fluid is introduced axially via the inlet section 28 and passes directly into the suction channel 2 of the pump.
The different components of the pump are known and their description will not be detailed further, with the exception of the axial balancing system of the pump part of which is visible in FIG. 2.
This axial balancing device or axial thrust balancing device of the rotating assembly is integrated with the impeller 11 and comprises a single rear balancing chamber 18 interposed between the rear part of the wheel 11 and a part of the stator 12, and a passage 20 connecting the rear balancing chamber to the fluid duct.
A balancing chamber is a chamber in which fluid pressure prevails, the action of this pressure on a mobile element (here, the rotor) acting to regulate and slave the position of said element mobile.
Via the passage 20, part of the fluid passing into the fluid duct is drawn off to feed the rear balancing chamber 18 situated on the rear face 16 of the wheel 11.
The front face of this wheel 11 receives the pressure of the fluid duct 14 which tends to move the rotor 10 to the rear. On the other hand, the fluid pressure in the rear balancing chamber 18 tends to move the rotor 10 to the front. Equilibrium is attained when these forces are compensated axially, the effort exerted by the fluid on the rotor 10 at the level of the rear chamber 18 compensating the axial impingement of forces exerted by the fluid on the other parts of the rotor during the various operating phases of the pump.
To reach and maintain this equilibrium, the axial balancing device modulates the feed pressure of the rear chamber 18 via the axial shift of the rotor, as follows:
The fluid-transfer passage 20 has a radial nozzle 40 extending between the rotating wall linked to the rotor 10, that is, to the centrifuge wheel 11, and a wall opposite the stator 12, that is, a fixed wall of the pump. The section for the fluid passage of this radial nozzle depends on the relative axial position of the rotor and of the stator: it increases when the rotor moves to the rear (increase in thickness A of fluid film passing through the passage 20, in FIG. 2, when the wheel 11 moves to the right), which causes an increase in the throughput entering the rear chamber, a rise in pressure in the rear chamber, and accordingly an increase in recoil force exerted by the fluid on the rotor tending to thrust it forwards. Inversely, if the rotor 10 tends to shift to the front (to the detriment of the thickness A of fluid film), the recoil force diminishes via an inverse mechanism, compelling the rotor to retreat more to the rear.
It is understood that the shift of the rotor modulates the pressure in the rear balancing chamber 18, and accordingly keeps the rotor in a substantially constant axial position, and advantageously with minimum friction. The system is auto-regulated, and tends to keep the rotor in its balanced position.
However, in such a thrust-balancing system, since the flow area of the fluid is intrinsically variable, the sole degree of liberty the designer has to vary the effect of the device, and therefore the pressure loss between the upstream and the downstream of this passage, is the radial distance B via which fluid flows into the nozzle of the passage, in this case corresponding to the spread between the internal and external radii of the crown formed by this nozzle.
Now, it is not necessarily to be desired to overly increase this distance B, and with it the dimension of the corresponding walls of the rotor and/or of the stator, as the pump is more sensitive to deformations of these pieces, because of the risk of harmful contact between the rotor and the stator.
Also, the increase in this distance B may not be enough to create the pressure loss necessary to equalise axial thrust, in particular in the case of a single-stage pump with open centrifuge wheel: In this particular case, the only surface on which fluid can be circulated is the balancing piston situated at the back of the wheel. In multi-stage pumps, each wheel can contribute to axial balancing of the pump.
By way of a known alternative solution, if the increase in width of the radial nozzle is insufficient to compensate axial forces, use can be made of an offset axial balancing plate. This brings with it an increase in the complexity of the pump and its execution, and a loss in yield for the pump. Also, especially because of the resulting elongation of the shaft, the axial and/or radial bulk of the pump is increased.